U.S. Pat. No. 3,284,529 describes the production of linear dimers of conjugated diolefin aliphatic compounds by bringing these compounds to a temperature of 70.degree. to 160.degree. C. in the presence of a zerovalent nickel catalyst derived from nickel carbonyl, and in the presence of a phenolic compound as co-catalyst, the said nickel catalyst being present in the proportion of 0.5 to 5% by weight of the said diolefin compound and the said phenolic co-catalyst being present in the proportion of 10 to 35% by weight of the said diolefin compound.